Super-K Monte Carlo: Detector Simulation

Monte Carlo Simulations

Particle physics experiments make frequent use of
Monte Carlo: detailed simulations of particle interactions and
the resulting detector signals, incorporating known physical
processes. Super-K is no exception. In Super-K, we simulate both
neutrino interactions and background events in order to compare
real data to expectation.

In general, particle physics experiment
simulations take place in two steps.

The first step employs a generator, or
an "interaction generator". A generator simulates the interaction
you are interested in, in the relevant material. For instance,
for neutrino experiments, "neutrino generator"
software simulates the charged particles that result
from interactions of neutrinos with the detector material
(say, water), given an input flux
of neutrinos. Super-K uses two different generator codes:
NUANCE, and NEUT.
The output of a neutrino generator is a list of "vectors":
it's basically a list of the particles that come out of a simulated
interaction. For each outgoing particle, the list contains, at
least: particle type, energy, direction, position (and sometimes
other info). This list of generated particles is also referred
to as a kinematics file.

The second step is detector simulation. This takes
kinematics files as input, and then simulates what the charged
particles in each event would look like in the actual detector you
have. Detector simulation software often makes use of packages like
Geant
(old, Fortran-based code), and Geant4 (a
newer, C++ based package). Simulation of a particle in a detector
is sometimes referred to as tracking, since the software
tracks the particle's energy loss and trajectory in the detector.

Super-K's standard detector simulation
software is called skdetsim and it employs old Geant 3.

In this example you will start at the detector simulation step,
and create by hand some kinematics files, and run them through
skdetsim to create some SK MC events.

Setup

Before starting, you'll need to have run the usual Super-K setup stuff.

Files for this example

sk4_odtune.card: a "card file". This contains
a list of parameters and options to be used by skdetsim.

test.kin: a sample kinematics file.

Look at test.kin. This will be used as input to
skdetsim. This contains the kinematics information (a
"vector") for one event to be simulated, in
Nuance format. It has:

$ begin
$ vertex 0. 0. 0. 0
$ track 13 5000. 0. 0. -1. 0
$ end

The $ begin and $ end lines tell skdetsim where
an event begins and ends (you can add more events to the same kinematics
file).

The $ vertex line indicates where the interaction
happened in the detector (and so is where the charged particles
should begin their trajectories-- their "tracks"). The first three arguments
are the x, y, z coordinates of this vertex.
In the Super-K spatial coordinate system (0,0,0) is the center
of the tank. So in the sample event to be simulated, the track
originates at the center of the detector. (The last argument is time, which you
can ignore for the moment.)

The $ track line indicates the
type and momentum of the particle to be tracked. In this example, there
is just one track (one particle in the event), but there
can be more than one particle per event.

The next argument is the total mass-energy of the particle, in MeV. In
this case we have a 5 GeV muon.

The next three arguments are the three components of the
(normalized) direction
vector of the particle. In SK coordinates, z goes up/down and
the x-y plane is horizontal. So in this example, the particle
is going straight down (-z direction).

The last argument should be 0 if
you want the particle to be simulated.

Running this example

myskdetsim.sh is the script to use to run skdetsim
in this example. It takes several arguments to indicate the input and
output files. Run it as follows:

./myskdetsim.sh sk4_odtune.card test.zbs test.kin

The first argument is the card file with options to skdetsim.

The second argument is the output file. In this example you will
be creating test.zbs containing the simulated event in ZBS format.
Note: if the output file exists already when skdetsim is
run, by default new simulated events are added to the end of it. So
you may want to delete it first, if you want a fresh output file.

The third argument is the input kinematics file.

After you have run this (it may take a bit of time to run), look
at the generated output file test.zbs with superscan.
Check that it indeed looks like a muon starting at the center of the
tank and going straight down.

Stuff to try

Create a new kinematics file and run it through skdetsim.
Try modifying the vertex, energy and direction of the muon
simulated, and check with superscan that your output file
makes sense (e.g. try 10,100,
500, 50000 MeV; try straight up, horizontal, at an intermediate angle;
try at various positions inside the tank).

Try adding new particles of different types to the event: electrons,
pions, protons, neutrons, gammas. Check with superscan that the
output makes sense.